Characterizing Historic ShakeMaps: A Tale of Two "SC" Earthquakes

Susan E. Hough

Published September 10, 2023, SCEC Contribution #12902, 2023 SCEC Annual Meeting Poster #191

U.S. Geological Survey ShakeMaps for current earthquakes are now generated using a combination of model predictions, recorded ground motions, and sometimes macroseismic intensities from the Did You Feel It? (DYFI) system. This same recipe can be followed for historical and early instrumental earthquakes (Allen et al., 2008), typically based on model predictions and interpreted macroseismic data. Where rupture models have been determined and carefully interpreted and detailed macroseismic intensities exist, historical ShakeMaps incorporating “Did They Feel It?” data and model predictions can mimic the first-order characteristics of modern maps. Historical ShakeMaps can reveal a surprising level of detail, and help confirm the consistency of intensity distributions with shaking predicted by modern models. For ShakeMaps to be realistic, it is key to have carefully interpreted intensities based on original sources. We present examples from two “SC” earthquakes, the 1933 Long Beach, California, earthquake, a pivotal event for Southern California, and the 1886 Charleston, South Carolina earthquake. The two maps draw respectively from rupture models derived from kinematic source modeling (Hough and Graves, 2020) and recently developed elastic deformation modeling based on identified geodetic constraints (Bilham and Hough, 2023). Notably, both maps illustrate excellent consistency between shaking predicted by modern ground motion models (GMMs) and intensities determined from extensive macroseismic data. Especially severe damage in Compton in 1933 is not predicted by GMMs, but the modeling by Hough and Graves (2020) shows that especially high accelerations and severe damage can be explained in part by 3-D focusing effects. For Charleston, estimated intensities are consistent with NGA-East GMMs using the preferred estimated magnitude, 7.3. This earthquake caused not only severe damage in the epicentral region, but due to the combination of low attenuation and amplification on Atlantic Coastal Plains sediment, caused damage to distances of 400-500 km.

Citation
Hough, S. E. (2023, 09). Characterizing Historic ShakeMaps: A Tale of Two "SC" Earthquakes. Poster Presentation at 2023 SCEC Annual Meeting.

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